This study will examine how two thyroid preparations, levothyroxine (T4) and liothyronine (T3), affect subcutaneous adipose tissue in normal subjects. Thyroid hormone plays an important role in the maintenance of energy metabolism. Specifically, hyperthyroidism is associated with weight loss via an increase in metabolic rate and lipolysis. While this phenomenon has been studied in vitro quite extensively, very little information is available on the local action of thyroid hormone on adipose tissue in vivo and, in particular, at the present no information is available on the role of peripheral conversion of thyroid hormone in subcutaneous adipose tissue. We thus propose to study the local action of thyroid hormones on adipose tissue using microdialysis, a minimally invasive technique that allows the serial determination of components of the extracellular fluid. The measurement of glycerol after correction for changes in blood flow will be used to evaluate the effects of the thyroid hormones on local lipolysis.